PROPAGATION OF NITRIC-OXIDE POOLS DURING CONTROLLED MECHANICAL VENTILATION

Objective. Infusing nitric oxide at a constant rate into a breathing c ircuit with intermittent mainstream flow causes formation of nitric ox ide pools between successive breaths. We hypothesized that incomplete mixing of these pools can confound estimates of delivered nitric oxide concentrations. Methods. Nitric oxide flowed at a constant rate into the upstream end of a standard adult breathing circuit connected to a lung model. One-milliliter gas samples were obtained from various site s within the breathing system and during various phases of the breathi ng cycle. These samples were aspirated periodically by a microprocesso r controlled apparatus and analyzed using an electrochemical sensor. R esults. The pools of nitric oxide distorted into hollow parabolic cone shapes and remained unmixed during their propagation into the lungs.I n our preparation, time-averaged nitric oxide concentrations were mini mal 60 cm downstream of the infusion site (18 ppm) and maximal 15 cm u pstream of the Y-piece (36 ppm). The concentrations were mid-range wit hin the lung (23 ppm), yet were substantially less than predicted by a ssuming homogeneity of the gases (31 ppm). Generally, nitric oxide con centrations within the lung were different from all other sites tested . Conclusion. Incomplete mixing of nitric oxide confounds estimates of delivered nitric oxide concentrations. When nitric oxide is infused a t a constant rate into a breathing circuit, we doubt that any sampling site outside the patient's lungs can reliably predict delivered nitri c oxide concentrations. Strategies to ensure complete mixing and repre sentative sampling of nitric oxide should be considered carefully when designing nitric oxide delivery systems.